The consumption of wood has outpaced the growth of trees. One way to remedy the shortage of solid lumber is to make plywood. Plywood generally consists of several layers of wood veneers. The number of layers of plywood and thickness of veneers vary with different applications. For example, the plywood used for hardwood flooring generally has either 3-ply or 5-ply construction. The thickness of the veneer ranges from 1/32 inch to 3/8 inch. Wood veneer layers are laminated together with adhesive to form the solid wood composite plywood.
A wide variety of adhesives have been commercially used as wood bonding adhesives including phenol-formaldehyde, phenol-resorcinolformaldehyde, urea-formaldehyde, urea-melamine, melamineformaldehyde, polymeric diphenylmethane diisocyanate, water based polyvinyl acetate homo polymer or copolymer, tannin-based adhesive, and lining-based adhesive.
Polyurethane adhesives have also been used as wood to wood bonding adhesives. For example, Japanese patent 51140936 Dai Nippon describes urethane adhesives for bonding wet plywood veneer having moisture levels above 10%. This adhesive requires 3 to 5 hours at 80.degree.-90.degree. C. to react to form a polyurethane prepolymer which is melted and optionally dissolved in methyl ethyl ketone solvent before use. The Japanese patent 56104980 polyurethane adhesive contains polyvinylpyrrolidone and is used to bond wooden veneer irrespective of the water content in veneer. Japanese Patent 2147685 describes a two component polyurethane adhesive suitable for bonding wood at room temperature providing thermally stable, dry and wet adhesion. Japanese Patent 3244687 identified a two component adhesive containing polyol as one component and isocyanate as another component. This two component room temperature curable adhesive is suitable for adhering aggregated wood material and has heat and water resistance. Japanese Patent 6143202 describes a moisture curable adhesive with talc filler for bonding plywood by cold pressing plywood first, followed by hot pressing. Japanese patent application 50 [1975]-157504 describes a two-component polyurethane adhesive for use on wet undried wood veneer. The first component is isocyanate terminated prepolymer, the second component is a polyol curing agent. The water content of the veneer was 30% or greater and sometimes reached 100-150%.
Commercially available moisture curable polyurethanes have been used as laminating adhesives to bond plastic sheet, metal sheet, foam sheet and particle board to plywood. Moisture curable polyurethane adhesives have also been used to bond wood veneers to make plywood with a cold press process.
Most hardwood floorings are made from different numbers of layers of wood veneers; the most common is 3 layer or 5 layer construction. Each slice of wood veneer has a thickness of 1/32 inch to 3/8 inch. Wood veneer has various levels of moisture and may have 2, 4, 6, 8, 10, 12% or higher. The construction of plywood can consist of different wood veneers. Within one laminate, for example, one popular wood flooring is maple-poplar-maple veneer. The commercially available adhesives require a cure time upwards from 4 minutes, with 200.degree. F. (93.3.degree. C. ) press temperature. Furthermore, if the adhesives contain water, the press temperature will be further restricted to below 250.degree. F. (121.1.degree. C. ) to prevent an explosion from over-and sudden-heating water.